Method for speeding up synchronization by having a search frequency list comprising a limited number of search frequencies

ABSTRACT

A method for initial synchronization in a mobile telecommunications network to connect a mobile terminal ( 10 ) to a base station ( 12 ). The mobile terminal has a search frequency list comprising a limited number of search frequencies. The method comprises the step of searching for an available frequency in the order of the search frequency list. The search frequency list further comprises count values, indicating how frequently the corresponding frequency has been used, wherein the order of at least some of the search frequencies depends on the count values.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is for entry into the U.S. national phase under §371for International Application No. PCT/IB02/01399 having an internationalfiling date of Apr. 26, 2002, and from which priority is claimed underall applicable sections of Title 35 of the United States Code including,but not limited to, Sections 120, 363 and 365(c).

TECHNICAL FIELD OF INVENTION

This invention relates to a method for initial synchronization in amobile telecommunications network to connect a mobile terminal to a basestation, the mobile terminal having a search frequency list comprising alimited number of search frequencies, the method comprising the step ofsearching for an available frequency in the order of the searchfrequency list. The invention also relates to a mobile terminal.

BACKGROUND ART

In a telecommunications network according to the above each operatorbelongs to a public land mobile network (PLMN), and a user's ownoperator is called a home public land mobile network (HPLMN). The usercan visit other PLMNs and there are mechanisms to exchange charging dataso that the user can be billed correctly. In UMTS (Universal MobileTelecommunications System), the charging mechanisms have been specifiedby 3GPP (Third Generation Partnership Project). When a user switchesfrom one PLMN to another PLMN, this is called roaming.

One operator typically operates on 2 or 3 frequency carriers. Eachcarrier will only support one operator. In every cell there can beseveral frequency carrier belonging to different operators.

In order for a mobile terminal, for example a mobile phone, to initiallyfind a cell and a frequency carrier in WCDMA (wideband code-divisionmultiple access, the access method in UMTS) an initial cell selectionprocedure is performed.

The procedure is described in “Introduction to 3G mobile communication”by Juha Korhonen and 3GPP doc 25.214 Annex C, and includes the followingsteps from the mobile phone's point of view:

Initial Cell-selection Procedure

Slot Synchronization

1) Search for primary synchronization channels (P-SCH). All P-SCHs havethe same fixed primary synchronization code.

2) Once such a channel is found, acquire time slot synchronization fromit. This is typically done with a single matched filter (or any similardevice) matched to the primary synchronization code which, as mentionedabove, is common to all cells. The slot timing of the cell can beobtained by detecting peaks in the matched filter output.

Frame Synchronization and Code-group Identification

3) Acquire frame synchronization from the corresponding secondarysynchronization channels (S-SCH). This is done by correlating thereceived signal with all possible secondary synchronization codesequences, and identifying the maximum correlation value. Since thecyclic shifts of the sequences are unique, the code group as well as theframe synchronization is determined.

Scrambling-code Detection

4) Acquire the primary scrambling code from the corresponding CPICH(common pilot channel).

5) Decode system information from the cell to check whether it is asuitable cell for camping (i.e. it contains the right PLMN code andaccess to it is allowed). The mobile phone determines the exact primaryscrambling code used by the found cell. The primary scrambling code istypically identified through symbol-by-symbol correlation over the CPICHwith all codes within the code group identified in the second step.After the primary scrambling code has been identified, the Primary CCPCH(common control physical channel) can be detected, and the system andcell specific BCH (broadcast channel) information can be read. If themobile phone has received information about which scrambling codes tosearch for, the procedure can be optimised.

The procedure is executed on one carrier frequency at a time. The mobilephone uses a frequency list which can be a full frequency list with allpossible frequencies. The procedure is searching for a suitablefrequency in the list. A problem with the frequency list is that thesearching can be time consuming.

SUMMARY OF THE INVENTION

An object of the invention is to provide a general method to speed upand improve initial synchronization.

The invention provides, according to a first aspect, a method forinitial synchronization in a mobile telecommunications network toconnect a mobile terminal to a base station, the mobile terminal havinga search frequency list comprising a limited number of searchfrequencies, the method comprising the step of searching for anavailable frequency in the order of the search frequency list, thesearch frequency list further comprising count values, indicating howfrequently the corresponding frequency has been used, wherein the orderof at least some of the search frequencies depends on the count values.

The search frequency list is used in initial synchronization or when themobile terminal is roaming and tries to find HPLMN.

The search frequency list may be formed in the main processor anddelivered to and used in the physical layer.

By searching in the order of the count values the more frequently usedfrequencies and therefore more likely frequencies are searched first. Anadvantage with this is that it speeds up the initial synchronization.

In one embodiment of the invention the step of searching for anavailable frequency is performed during a maximum time for eachfrequency in the search frequency list, said maximum time being weightedin accordance with the corresponding count value.

An advantage with this is that more time is spent on searching for morelikely frequencies. This speeds up the search.

In another embodiment of the invention the first frequency of the searchfrequency list is the last one used.

The last frequency used is the most likely frequency to be used again,and therefore this frequency is first in the search frequency list.

In one embodiment of the invention the search frequency list comprises acombination of a HPLMN frequency list and a Roaming PLMN frequency list.

The HPLMN frequency list is controlled by the mobile terminal. Themobile terminal can start with a search frequency list which has HPLMNand Roaming PLMN (RPLMN) frequencies included. The frequencies in HPLMNand Roaming PLMN frequency lists are the most likely frequencies to beused.

In one embodiment of the invention the first frequency of the searchfrequency list is the last one used, the next frequencies are thefrequencies in the HPLMN frequency list sorted by the count values andthe following frequencies are the frequencies in the Roaming PLMNfrequency list sorted by the count values.

This list provides a very efficient way of finding the searchedfrequency.

In another embodiment of the invention the HPLMN frequency listcomprises time stamps, which indicate when the corresponding frequencieswere last used.

In one embodiment of the invention the method further comprises the stepof using the time stamps to determine which HPLMN frequency is to beremoved from the HPLMN frequency list due to a shortage of space.

In this way the HPLMN frequency least likely to be used is removed fromthe list when a new frequency is added.

In another embodiment of the invention the determined HPLMN frequency tobe removed is the one with the oldest time stamp.

A frequency with the oldest time stamp is the frequency used longest wayback in time and thus the frequency in the HPLMN frequency list probablyleast likely to be used again. This frequency is therefore selected andremoved from the list when the list is full and a new frequency is to beadded.

In one embodiment of the invention the Roaming PLMN frequency listcomprises time stamps, which indicate when the corresponding frequencieswere last used.

In another embodiment of the invention the method further comprises thestep of using the time stamps to determine which Roaming PLMN frequencyis to be removed from the Roaming PLMN frequency list due to a shortageof space.

The time stamps in the Roaming PLMN frequency list are used as in theHPLMN frequency list.

In another embodiment of the invention the determined Roaming PLMNfrequency to be removed is the one with the oldest time stamp.

In one embodiment of the invention the HPLMN frequency list and theRoaming PLMN frequency list comprise count values, the method furthercomprising the step of incrementing a count value in idle mode and intraffic mode when a corresponding frequency is used.

In another embodiment of the invention said mobile telecommunicationsnetwork uses Wideband CDMA as access method.

According to a second aspect, the invention also involves a computerprogram product directly loadable into a memory of a processor, wherethe computer program product comprises program code for performing themethod according to any of the embodiments described above when executedby said processor.

According to a third aspect, the invention also comprises a mobileterminal comprising a search frequency list with a limited number ofsearch frequencies, the mobile terminal being arranged to search for anavailable frequency in the order of the search frequency list so as tosynchronize with a base station in a mobile telecommunications network,wherein the search frequency list further comprises count values,indicating how frequently the corresponding frequency has been used,wherein the order of at least some of the search frequencies depends onthe count values.

The mobile terminal may for instance be a mobile phone, a personaldigital assistant, a laptop computer with a GSM or UMTS interface, etc.

In one embodiment of the invention the mobile terminal is arranged tosearch for an available frequency during a maximum time for eachfrequency in the search frequency list, said maximum time being weightedin accordance with the corresponding count value.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described inmore detail, reference being made to the enclosed drawings, in which:

FIG. 1 is a schematic front view of a mobile telephone.

FIG. 2 is a schematic block diagram of some components, with in thecontext of the present invention, of the mobile telephone shown in FIG.1.

FIG. 3 is a schematic illustration of a HPLMN frequency list, a roamingPLMN frequency list and a search frequency list according to the presentinvention.

FIG. 4 is a schematic block diagram for the purpose of explaining thesynchronization search process.

FIG. 5 is a schematic flowchart of one realization of weighting in thesynchronization search process.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 shows a mobile terminal and more specifically a mobile telephone10. The mobile telephone may be any commercially available device for amobile telecommunications system such as GSM, UMTS or D-AMPS.

As is well known in the technical field, the mobile telephone 10comprises an antenna 20, a loudspeaker 21, a display 22, a firstplurality of navigation keys 23, a second plurality of alphanumeric keys24, and a microphone 25.

FIG. 2 illustrates some internal components, within the context of thepresent invention, of the mobile telephone 10. A controller 41 isresponsible for the overall operation of the mobile telephone 10 and ispreferably implemented by any commercially available CPU (“CentralProcessing Unit”), DSP (“Digital Signal Processor”) or any otherelectronic programmable logic device. The controller 41 is coupled to aradio interface 20, 40, comprising the antenna 20 and radio circuitry40. The radio interface 20, 40 is responsible for establishing andmaintaining a wireless link 11 to the base transceiver station 12. As iswell known to a man skilled in the art, the radio circuitry 40 comprisesa series of analogue and digital electronic components, which togetherform a radio receiver and transmitter. The radio circuitry 40 comprises,i.e., bandpass filters, amplifiers, mixers, local oscillators, lowpassfilters, AD converters, etc.

The controller 41 is also connected to an electronic memory 42, such asa RAM memory, a ROM memory, an EEPROM memory, a flash memory, or anycombination thereof. The memory 42 is used for various purposes by thecontroller 41, one of them being for storing data and programinstructions, which form a man-machine interface 45. The man-machineinterface 45 also involves a keypad 43 (corresponding to the keys 23, 24in FIG. 1) and a display 44 (corresponding to the display 22 in FIG. 1).A user 46 of the mobile telephone 10 will operate the telephone throughthe man-machine interface 45, as is well known per se.

The mobile phone 10 comprises a HPLMN frequency list 100, shown in FIG.3. The HPLMN frequency list 100 is a list of HPLMN frequencies 101,denoted F1, F2, F3 and F4. The HPLMN frequencies 101 are sorted by thecount values 102. Each count value 102 is incremented by one when thecorresponding frequency 101 is used in idle mode or in traffic mode. Thecount value 102 may be limited to a MAX_COUNT_VALUE of 1000. The HPLMNfrequency list 100 also comprises time stamps 103, which indicate whenthe corresponding HPLMN frequency 101 was last used. The time stamps 103are used when the frequency list is full and a new frequency is going tobe added. The HPLMN frequency F2 earliest used is then removed from theHPLMN frequency list 100, and the new frequency is added to the list.

The mobile phone 10 also comprises a Roaming PLMN frequency list 110which is a list of frequencies F′1, F′2, F′3 and F′4, that were in usein a PLMN where the mobile phone 10 was roaming. The roaming PLMNfrequencies 111 are sorted by count values 112. Each count value 112 maybe limited to a MAX_COUNT_VALUE of for example 1000. The Roaming PLMNfrequency list 110 also comprises time stamps 113, which have the samefunction as in the HPLMN frequency list 100.

The initial synchronization comprises the initial cell-selectionprocedure described in the background section. The initialcell-selection procedure is executed in the physical layer, and the RRC(Radio Resource Control) layer has only a management role.

The procedure is performed on one carrier frequency at a time and in thefrequency order of a search frequency list 120, until a suitablefrequency is found.

The search frequency list 120 is a short frequency list used in initialsynchronization. It can also be used when the mobile phone 10 is roamingand tries to find HPLMN.

The search frequency list 120 comprises search frequencies 121 withcorresponding count values 122 and is built up in the main processor.The first search frequency F2 of the search frequency list 120 is thelatest one used. The next search frequencies F1, F3 and F4 are retrievedfrom the HPLMN frequency list 100. The next frequencies F′1, F′2, F′3and F′4 are retrieved from the Roaming PLMN frequency list 110. Thecorresponding count values 102, 112 are also added in the searchfrequency list 120.

The time spent on searching for a certain frequency among the searchfrequencies 121 is weighted with the corresponding count values 122. Forexample, the search time for a search frequency 121 can be calculated bymultiplying a predetermined max search time, for example 50 ms, with thecorresponding Count Value 122 divided by the MAX_COUNT_VALUE (=1000). Bythis a longer time is spent searching for a more frequently usedfrequency than the time spent on searching for a more rarely usedfrequency. For example, the time spent on searching for F2 is 50ms*204/1000=10.2 ms and for F3 is 50 ms* 23/1000=1.15 ms.

The mobile phone 10 repeats this procedure until the first suitablecell/frequency is found for camping. Once the mobile phone 10 hasmanaged to camp onto a cell, it decodes the system information.

FIG. 4 shows a general block diagram of one embodiment in which thecontroller 41, shown in FIG. 2, is implemented by a DSP (Digital SignalProcessor) 50. The DSP controls the physical layer blocks which performthe actual search operation. The DSP 50 is communicating with a MCU(Main Control Unit) 51 and a receiver 52.

The DSP 50 controls the initial cell-selection procedure comprising theslot synchronization 53, frame synchronization and code-groupidentification 54; and scrambling code detection 55.

FIG. 5 is a flowchart explaining one realization of the synchronizationprocedure which is as follows. In step 200 the DSP 50 receives thesearch frequency list 120 from the MCU 51. In step 210 the DSP 50selects the first search frequency 121 in the search frequency list 120.A count number I is initially set to zero. In step 220 the DSP sets aweighting number N based on the count value 122 corresponding to thefirst search frequency 121. For example, N can be calculated bymultiplying a maximum value of N, for example 100, with the currentcount value 122 divided by the MAX_COUNT_VALUE (=1000). N is then theinteger value of this result and can be the number of times the DSPtries to find this current search frequency before trying the nextsearch frequency in the search frequency list 120. For example, for thefirst search frequency according to the search frequency list 120 inFIG. 3, N may be calculated by 100*204/1000=20.4. N is then 20.

In step 230 the DSP starts the slot synchronization. In step 240 the DSPstarts the frame synchronization and the code-group identification. TheDSP then starts, in step 250, scrambling code identification. In step260 a check is made to control if the scrambling code is detected. Ifthe scrambling code is detected the DSP sends, in step 270, a signal tothe MCU 51, which signal comprises the information that thesynchronization is OK. If the scrambling code is not detected the countnumber I is increased by one, and in a step 280 the weighting number Nis compared to the first count number I. If the first count number I isnot larger than N, a new attempt to find the scrambling code with thesame search frequency as before is made. For the first frequency thisloop can be performed 21 times before I>N.

If the scrambling code has not been detected and I is larger than N, theDSP selects, in step 290, the next search frequency 121 in the searchfrequency list 120 and continues the flow chart. The count number I isset to zero, the DSP sets the second count value N based on the secondsearch frequency 121 and the loop continues. For the second frequency,100*235/1000=23,5 and N is decided to be 23.

If no suitable frequency is found the mobile phone has to search everyfrequency in the system. The memory 42 usually maintains this kind oflists.

WCDMA is typically an asynchronous system, but if relative timing of thebase stations is signalled to the mobile phone, then the mobile phonecan skip the cell search procedure when performing neighbour cellmeasurements for cell reselection/handoff.

The foregoing embodiment of the present invention provides an exemplaryillustration and description, but is not intended to be exhaustive or tolimit the invention to the precise form disclosed. Modification andvariations are possible in light of the above teachings or may beacquired from practice of the invention.

For example the mobile terminal 10 can also be a personal digitalassistant, or a laptop computer with a UMTS interface. Furthermore thesearch list can be built up in other ways depending for example of itssize.

1. A method, comprising: providing a search frequency list comprising aplurality of search frequencies and a count value for each frequency ofsaid plurality of search frequencies indicating how frequently eachfrequency has been used, and searching for an available frequency in theorder of the search frequency list, wherein the order of at least someof the search frequencies depends on the count values, and whereinsearching for an available frequency is performed during a maximum timefor each frequency in the search frequency list, said maximum time beingweighted in accordance with the corresponding count value.
 2. The methodaccording to claim 1, wherein the first frequency of the searchfrequency list is the last one used.
 3. The method according to claim 1,wherein the search frequency list comprises a combination of a homepublic land mobile network frequency list and a Roaming public landmobile network frequency list.
 4. The method according to claim 3,wherein the first frequency of the search frequency list is the last oneused, the next frequencies are the frequencies in the home public landmobile network frequency list sorted by the count values and thefollowing frequencies are the frequencies in the Roaming public landmobile network frequency list sorted by the count values.
 5. The methodaccording to claim 3, wherein the home public land mobile networkfrequency list comprises time stamps, which indicate when thecorresponding frequencies were last used.
 6. The method according toclaim 5, further comprising using the time stamps to determine whichhome public land mobile network frequency is to be removed from the homepublic land mobile network frequency list due to a shortage of space. 7.The method according to claim 6, wherein the determined home public landmobile network frequency to be removed is the one with the oldest timestamp.
 8. The method according to claim 3, wherein the Roaming publicland mobile network frequency list comprises time stamps, which indicatewhen the corresponding frequencies were last used.
 9. The methodaccording to claim 8, further comprising using the time stamps todetermine which Roaming public land mobile network frequency is to beremoved from the Roaming public land mobile network frequency list dueto a shortage of space.
 10. The method according to claim 9, wherein thedetermined Roaming public land mobile network frequency to be removed isthe one with the oldest time stamp.
 11. The method according to claim 3,wherein the home public land mobile network frequency list and theRoaming public land mobile network frequency list comprise count values,the method further comprising incrementing a count value in idle modeand in traffic mode when a corresponding frequency is used.
 12. Acomputer readable medium encoded with a computer program product,wherein the computer program product comprises program code forproviding a search frequency list comprising a plurality of searchfrequencies and a count value for each frequency of said plurality ofsearch frequencies indicating how frequently each frequency has beenused, and searching for an available frequency in the order of thesearch frequency list, wherein the order of at least some of the searchfrequencies depends on the count values, and wherein searching for anavailable frequency is performed during a maximum time for eachfrequency in the search frequency list, said maximum time being weightedin accordance with the corresponding count value.
 13. An apparatus,comprising: a search frequency list comprising a plurality of searchfrequencies and a count value for each frequency of said plurality ofsearch frequencies indicating how frequently each frequency has beenused, and a controller configured to search for an available frequencyin the order of the search frequency list so as to synchronize with abase station in a mobile telecommunications network, wherein the orderof at least some of the search frequencies depends on the count values,and wherein the controller is configured to search for an availablefrequency during a maximum time for each frequency in the searchfrequency list, said maximum time being weighted in accordance with thecorresponding count value.
 14. An apparatus, comprising: a searchfrequency list comprising a plurality of search frequencies and a countvalue for each frequency of said plurality of search frequenciesindicating how frequently each frequency has been used, and means forsearching for an available frequency in the order of the searchfrequency list so as to synchronize with a base station in a mobiletelecommunications network, wherein the order of at least some of thesearch frequencies depends on the count values, and wherein theapparatus further comprises means for searching for an availablefrequency during a maximum time for each frequency in the searchfrequency list, said maximum time being weighted in accordance with thecorresponding count value.
 15. The apparatus according to claim 13,wherein a first frequency of the search frequency list is the lastfrequency that was used.
 16. The apparatus according to claim 14,wherein a first frequency of the search frequency list is the lastfrequency that was used.
 17. The apparatus according to claim 13,wherein said apparatus is configured to operate as part of a mobileterminal.
 18. The apparatus according to claim 14, wherein saidapparatus is configured to operate as part of a mobile terminal.
 19. Themethod according to claim 1, further comprising performingsynchronization in a mobile telecommunication network.
 20. The methodaccording to claim 19, wherein said mobile telecommunication networkuses wideband code-division multiple access as an access method.